HV power line modeling in demand of a measuring system development

ATTILA KMENT, MAREK PÍPA, ŽANETA ELESCHOVÁ, ANTON BELÁŇ Department of electrical power engineering

Faculty of electrical engineering and information technology of Slovak university of technology Ilkovičova 3, 812 19 Bratislava

SLOVAK REPUBLIC attila.kment@stuba.sk, marek.pipa@stuba.sk, http://www.kee.fei.stuba.sk

Abstract: - Contribution deals with modeling of a high-voltage power line. The model may be effective by the further development of an automated measuring system based on Rogowski principle. On the one hand is necessary to verify the evolving measuring system’s current transfer-characteristic density, on the other hand the model has to ensure safe in-service operation. One of possible solutions can be the galvanic separation of measuring unit from indication device. Key-Words: - Rogowski coil, High-voltage, High-current. 1 Introduction The effort to replace instrument current transformers with precision flexible AC current probe based on Rogowski principle in power system is conditioned by qualities or properties of the new measuring system. The most important one is the current transfer characteristic invariance in strong electric field, what have to be verified in lab conditions. To fulfill this objective is necessary to build up a workplace for the needs of equipment research, which may be applicable in wide range of applied voltage. As the level of applied voltage rises, the question of galvanic separation of measuring and indication units becomes essential for the functional safety. 2 Workplace build-up Initially a reversely connected instrument current transformer in conjunction with a high-voltage transformer was considered as a source of high-current at high-voltage level. Because the instrument current transformer was impossible to obtain, it was necessary to design an alternative wiring. The suggested design concept of workplace, more precisely its wiring schematic is shown in Fig.1. Current transformer (type Elektrovod Jevišovice TS-24/6) marked by a letter T in Fig.1, is supplied by a booster B (type TuR Dresden REOgs 32/380-1M). A short-circuited middle-voltage cable (NA2XS(F)2Y 1x300RM/25 - 18/30 kV) was chosen as a secondary winding of the current transformer (R letter in Fig.1). The reason for this selection is, that the selected cable is insulated by its working insulation (30 kV AC between the cable core and cable shielding, stated by the manufacturer) from current transformer. The

shielding at one end of the cable is grounded. The second end’s shielding is cut, which warrants the voltage-protection of current transformer. Both ends of the skinned core of the cable are connected. At the connection is applied high-voltage from high voltage transformer HT (type TuR Dresden GPT 6/12,5). The current loading capacity of loop circuit is limited by the type of high-voltage cable used, more precisely by prescribed maximum core temperature. The maximum applicable voltage is defined by the cable insulation type.

Fig.1 Circuit diagram of suggested workplace The measuring point of designed workplace is the non-insulated core of applied high-voltage cable. Fig.2 shows the real connection of devices, when the measurement of current transfer-characteristic consistency took place. At this point, no high voltage was applied. To demonstrate the functionality of the research equipment according to its current range a series of comparative measurements were accomplished by using ammeter clamp (Metra PK-310) simultaneously with the new measuring system based on Rogowski

Proceedings of the 9th WSEAS International Conference on POWER SYSTEMS

ISSN: 1790-5117 46 ISBN: 978-960-474-112-0

coil. The applied current range was chosen by respecting the short-time current loading capacity of selected circuit, and the maximum ranges of measuring devices. A maximum current value achieved during this test was 2kA. The temperature of cable core was continually controlled by a digital thermometer to ensure, that the maximum allowed overload temperature was not exceeded. The observed difference between both ammeters is not bigger than 2 % from the range. The ammeter clamp and the current sensor cable, a flexible inductive coil, are shown in the upper detail of Fig.2. The lower detail shows the electronic box of evolving precision flexible AC current probe based on Rogowski principle.

Fig.2 Workplace set up with details 3 High-voltage measurements After the verification of scheme’s functionality from the view of current supply, applying of high-voltage followed. The maximum allowed voltage of the measuring device in touch with a non-insulated conductor is 600 V. To reach a higher value, the current sensor cable had to be taken away from the surface of non-insulated cable core. The maximum applied voltage was 3 kV, because in current stage of development the producer of current sensor cable states this voltage as maximum allowed value. Fig.3 shows the arrangement used by voltage tests. The voltage was measured by a 30 kV voltage probe in conjunction with a digital multimeter (HP 947A). According to the ambition of producer in regard to increase the maximum allowed in-service voltage, a series of measurement is under consideration in wider range of applied voltage, ideally at all voltage-levels of

power system. Then an upgrade of existing, or a new workplace design will be necessary. If the in-service voltage-increase of evolving equipment is under consideration, a galvanic separation between measuring and indication or evaluation units seems to be a necessity. One of possible solution may be usage of two optical cables. One of them could be used to supply the media converter and the electronic box, which needs a voltage of 3V for reliable operation, and the other one could assure data communication. Usage of regular, on-the-market available optical cables is considered. The problem is that the producer of mentioned optical cables does not state the recommended intensity of electric field, where these cables are able to operate properly.

Fig.3 High-voltage measurement Deploying a research in this field may provide some valuable information about the selected optical cables. Primarily was decided to verify the simplest arrangement of two equally long optical cables with their terminals on (shown in Fig. 4). Before providing withstand test by applied AC high-voltage, the insulation resistance measurement took place by using the MEGGER S1-1052 insulation tester. The resistance evaluated by applied 10 kV DC was more than 500 GΩ. Breakdown voltage of 1m long optical cable pair was determined by 5 measurements to be 360 kV in relation to the earth under normal atmospheric conditions. Insulation resistance measurements ended with the same result as

Proceedings of the 9th WSEAS International Conference on POWER SYSTEMS

ISSN: 1790-5117 47 ISBN: 978-960-474-112-0

4 Conclusion those carried out before withstand testing. This fact proves that either within the cable or on the surface of selected optical cable no conductive path was formed.

The main aim of the designed workplace is to enable to performing of necessary tests at the evolving measuring system by applied high current and voltage simultaneously. It is also important to develop a galvanic separation between components of measurement system at high-voltage level and at the ground potential. This separation may be carried out by using desirable optical cables. Attention is also given to the future problem-solving approach of presented research.

Specimen was during withstanding tests damaged at their terminal sides as illustrates Fig. 5.

Acknowledgement Contribution arose during the work on project APVV-0546-07. References STN EN 60060-2 High voltage test techniques – Part 2: Measuring systems ŠANDRIK, P. Skúšobníctvo a diagnostika: Prednáškové texty, Bratislava, FEI STU, 2004 STN EN 60865-1 Short-circuit currents. Calculation of effects. Definitions and calculation methods.

Fig.4 Withstand voltage test Presented results were obtained by using new 1m long optical cables as specimen. In the future measurements under different atmospheric conditions, different arrangements of specimen, long-time aging tests and leakage current measurements will be performed. These tests are needed to accomplish for different kinds and lengths of optical cables. A series of new specimen is already prepared to be tested in our laboratory. The workplace for providing long-time aging tests is prepared as well.

Fig. 5 Damaged terminals of specimen

Proceedings of the 9th WSEAS International Conference on POWER SYSTEMS

ISSN: 1790-5117 48 ISBN: 978-960-474-112-0